Electrical voice transmission network



July 26, 1932. T. H. TURNEY 1,869,031

ELECTRICAL VOICE TRANSMISSION NETWORK Filed March 7, 1930 Imm-r qreunr fljh Timmv H11-M Patentes July 26, 1932 UNITED STATES rnnvoa PATENT foil-FICE TBIC COMPANY, F LIVERPOOL, ENGLAND, A BRITISH COMPANY nLnc'raIcaL voren 'rnansmssron Nnrwonx Application mcd umn '1, 1930, semi No. 4ss,s'71, and in creat. Bram nnen s, v192e.

The present invention relates to the construction of a new form of transmission unit which is applicableboth to filters .for electrical networks which may be, for instance,

l 5} of the' typeemployed in connection with vtherlnionic valve repeaters, or to cables used.

for transmitting current of speech frequency or carrier currents, and more particularly, ap lies to loaded cables.

t is well known that filters andv loaded cables as usually designed give rise to what are known as transients, that is. to say, disturbances which are produced when particular switching operations are effected. Wave lters are known in which the various sections have the same transition frequencies but havev dissimilar impedance character istics. v

According to the invention all sections of the transmission unit have the same nominal impedance but adjacent sections have different'transition frequencies. The advantages of a transmission unit according to the 1no vention are firstly, that the transmission unit can be designed to present fa sensibly constant impedance at its terminals over the entire ran e of transmission frequencies and possibly gives greater freedom from transients.

The invention will be better understood by referring to some extent to the theoreticaly considerations and also to the accom' anying y Figur -Zz is changed bv a factor. which es 1-4, which figures will be scribed in turn where they are referred to inthetext.

In general the propagation constant of a section of a filter closed by its correct impedances is given by cosh Pezzi* According to the inventionv Z1 is changed from lsection to section by a factor.r which may or may not be a constant factor, while v is always the reciprocal o vthat for Z1. y Thus -Figure 1 represents a convergent filter anda and b represent successive factors. The co-eiiicicnts aand b may be chosen in any way for specific purposes. For example yond. Secondly the transmission unit structure the filter, may converge to alcertainlpoint and afterwards diverge again. -The cut off frequency of the two ends of the structure may even be different.

r s an example,the invention may\be applied to low lpass filters in the manner illustrated in Figure 2. 1

' I t is assumed that it, isY required ,t0V (iesign a low pass filter tofgive a cut-off frequency of 800 cycles, Vtyhatis to say, that it will pass frequencies ofless than 800 cycles and cut off-higher frequencies. To do this" 5 The cutoff frequency Henceit willbe readily appreciated how' the sections are to be'calcu ated. Q

It will be understood thatwhile it is as?.

sum'ed that the various sections have the same nominal impedance, this is by no means essential and it will-be possible for the impedance as lwell as the cut-'off frequency to e changed progressively, enabling the p to operate between unequal impedances. "l

The invention could also be applied yto band 'pass filters, in which case the outer sections would permit of the passage of a'- widel band of frequencies and successive sections would permit graduallyy reduced bands, the band towards the middle being the leastand corresponding to the band? of. frequencies which the filter is designed to pass.

' The reduction of transients with 'a filter fact' that these disturbances are due chiefly to phase-delay dierences. With a filter havof each section isrepresented by theformula :v f

according to the inventionqis basedon thei" i scope of the invention to that just described.

or frequencies, the same delay is concentrated at or around certain definite parts of the range. With a filter embodying the princif ples o f the invention, however,`the delay will be distributed and thus the volume of the transient produced, for instance, by switching on, will be reduced.

An alternative design of filter within the assumed that it is desired to construct a low Acables is tos ace theco vvapart' and ow for a certain amount of pass filter to cut off 900 cycles and .have a nominal impedanceoi. 600 ohms. From the usual formula for regular filters '60m-Jg, and

'rw/L0 L=.212 henry and C=.59 mf. The next step to make up sections for which A becomes'larger. I

' This may be done, for example, by taking 20% off .212 ie. .1696. .A further 20% oli' .1696 gives .1356 henry. The values of the inductancesshould thenbe increased progressively in a similar manner.

In the present example C is so proportioned as to make the square root of L divided Vb C constant for each section, consequently t e valuesi'of C are. respectively .59 mf. .472 mf; and .3776 mf. These would then be as shown in Figure 3.

AAdjacent inductances could be combined into a sin le unit, forexample:

The in uctances .106+.085=.191 henry could be'wound on a singles olor coil.

While we .have shown a jacent sections having cut-off frequencies in ,constant ratio, the invention is not limited to this, the essential feature o'f the invention being that the cut-off frequencyofthe section changes from somevalu'e at onepart of the filter to a different value at anotherpart thereof.` vThe application to band-pass filters is similar, the band which is passed being wider atthe ends than in the middle..

The invention is also applicable to a loaded cable. The usual arran ement of loading s at even distances tapering at the ends.' This arrangement leads to transient effects which are more pronounced on long than on short lines. By applying the invention to a loaded cable a convergent cable can be obtained whose cutoii' frequency varies frompoint to point, being lower at one pointthan at others. `As an example a cable may have a low cut-olf frequency in the middle .or also at-one end.

To reduce reflection losses the nominal impedance of the line should be made constant. At present, long lines arefrequently lightly loaded so as to reduce transients giving a cutofi' frequency 'of say 5,000 cycles. A cable designed according to the invention would have a cut-off frequency which varied say from 3,000 cycles at the ends to 2,400 cycles in the centre.

The effect of having a high cut-off frequency at the ends would tend to give a more gradual variation of impedances with frequency when considering impedances from the cable ends.

It is well known that transients or building up times are proportional to the slope of the Wave length constant frequency curve, which quantity is frequently called:

Now, in a line which is loaded at regular intervals, the point of greatest wave length. constant for each loading section occursl at the same frequency and the slope for the whole cable is a maximum. By making the cut-ofi' frequency different for every loading section the slope would be greatly reduced.

One example of carrying out the invention will 'be described, giving the coil values and spacing for a typical line. It is necessary in the first case to decide on the cut-off frequency .of the end and centre sections. Secondly, to decide on the type'of taper, that is to sa whether the cut-oii' shall be in geometrical or arithmetical progression or shall follow some other rule. The particularlsystem employed is immaterial as concerns the invention itself.v

Thirdly, to select the ratio of variation of cut-off frequency from one section to another, suc'h that the cut-olf frequency changes from one decided value to the other in half the length of the cable. v

.Fourthly, knowing now the cut-oli frequenc for each section,'it is advisable to obtain e same value for for each section. Normally, thev section may be supposed to consist of two pieces of cable and one leading coil, or a portion of each t`wo coils and the intermediate piece of cable.

Havingfollowed these steps it becomes a' comparatively simple matter to design eachl section,` and the spacingand values ofthe coils can readily be determined.

It may at times be necessary to change both-l the cut-olf frequency and the nominal impedance f a cable from one end to the other, to make the cable work in between two por tions which have different impedances.

VReferring to Figure 4, we have given -anV example of a cable with variable cut-olf frequency. In this case the capacity of the length lA B and the inductance ofthe coil `L are decided by the required nominal impedance of the cable giving V N/E c y and the cut-off frequency of a particular sectionABgivingV v Thel impedance will be determined by the closest spacing possible which is permissible from the point of view of cost.v

What I claim as new and desire to secure l by LettersPatent is 1. A transmission element for. audio frel quencyor like electriccircuits comprising a number of sections, each section consisting'v of a wave filter having a different cut-0E frequency, the cut-oil' 'frequencies of adjacent sections increasing yprogressively from a certain frequency.

2. A A transmission element for electric circuits comprising aplurality of sections hav-- ing the same nominal impedance, each sec-V tion consisting of a wave filter having a different cut`oif frequency, the cut-off. frequencies of adjacent .sections varying progres' sively from one end of the element.

3. An electric wave filter consisting of a line composed ef-a plurality of sections, each section having an inducta'nce coil in series with the line and a condenser in shunt'across the line, said inductance coilsand condensers having-precomputed values so proportioned that each section has the same nominal impedance but a dilferent cut-olf frequency than the'adjacent sections. l

4. A transmission element comprising a loaded cable divided into a pluralitv' of sections,each section consisting of loading` coils and a different precomputed length of cable so that the cut-olf frequencies for adjacentl `sections vary progressivelyzalong the cable.,i

5. A transmission element comprising a loaded cable 'having loading coils spaced at -diier'ent distances apart, said cable being di'- vided into sections Ahaving thek same nominal impedance, each sectionconsisting of a CERTIFICATE OF CORRECTION.

Patent No. 1,869,031. v July 26, 1932. v

TREVOR HUGH TURNEY.

lt is hereby certified that the name of the assignee in the above nuinbered patent was erroneously described and specified as "Automatic Electric Company," whereas said name should have been described and specified as Automatic Electric Company Limited, as shown bythe records of assignments in this office; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office. Signed and sealed this 245th day of October, A. D. i932.

M. J. Moore, (Seal) .Acting Commissioner of Patents. 

